A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Diffraction based scatterometry within the semiconductor industry is up until now used mainly for overlay and critical dimension (CD) measurements within metrology.
In angular resolved scatterometry, a periodic mark on a substrate is simultaneously illuminated at various angles. The light diffracted by this mark is used to measure particular characteristics of that mark. If the period of the mark is sufficiently large, the diffracted light will contain higher diffraction orders. However, part of the first diffraction order is often mixed with part of the zeroth diffraction order, as shown in accompanying FIG. 5. This overlap of diffraction orders generally yields a less robust reconstruction of the characteristics of the mark. In order to separate out the different diffraction orders annular illumination can be used, and this results in separated zeroth and first order diffraction patterns as shown in FIG. 6. However, it has been found that the use of such annular illumination may lead to errors in the measured mark characteristics since annular illumination provides less information in the diffracted light. For example, in annular illumination, there are no light beams near normal incidence that also contain information that is valuable for measuring the mark characteristics.
In typical scatterometers, it is a problem that illumination is limited to apertures that are suitable for small pitches, for example smaller than approximately 1000 nm, if separation of the individual orders is required with sufficient pixels in the separated orders.
It is desirable to characterize alignment targets and also overlay targets with large pitch with an angular resolved scatterometer. However, alignment targets normally have gratings with pitches larger than 1000 nm. This means that the known apertures do not (or only partly) separate the diffraction orders. For such large pitches, as well as the first diffraction order, higher diffraction orders such as the second third, etc. may be detected. It would be desirable to make separation of orders possible while keeping the number of detector pixels for these orders sufficiently large for accurate alignment target asymmetry reconstruction.
One of the assumptions that are commonly made with the overlay calculations and CD reconstructions are that of a symmetrical grating which is also uniform across the entire wafer. This assumption of symmetrical gratings has been found through experimental data to be incorrect and a more comprehensive understanding of these grating asymmetries has been found to be necessary to produce more accurate results. A number of lithographic and in-line processing variables can lead to asymmetry both within the upper and lower target grating of an overlay grating stack. These asymmetries are convoluted within in the output overlay result and can have contributions from the bottom target grating asymmetry, the top target grating asymmetry and the relative overlay between the two targets. It would be desirable to de-convolute these single grating asymmetries from the actual relative overlay.
Furthermore, more accurate knowledge of grating asymmetry distributions across the wafer is more and more desirable for process control within semiconductor manufacturing.
Grating asymmetry affects accuracy on alignment, overlay and critical dimension measurements in diffraction based scatterometry. Current methods to detect this are both time consuming and/or destructive.
Grating asymmetries and their distributions across the wafer are a good indicator of process variations caused by various semiconductor manufacturing steps, for example Chemical Mechanical Polishing (CMP) and etch. A fast and easy to use method of detecting this phenomena and it's variation across the wafer is desirable.
There is currently no known method to feed forward information of grating asymmetry and its distribution across the wafer.
There is currently no known method of accurately, quickly and non-destructively characterizing grating asymmetry and its distribution across the wafer.